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Find out more on how to host your own Frontiers Research Topic or contribute to one as an author by contacting the Frontiers Editorial Office: researchtopics@frontiersin.org 2 October 2015 | Synaesthesia Frontiers in Psychology Topic Editors: Michael Banissy, Goldsmiths University of London, UK Roi Cohen Kadosh, University of Oxford, UK Clare Jonas, University of East London, UK Synaesthesia is a rare experience in which one property of a stimulus evokes a secondary experience that is not typically associated with the first (e.g. hearing words can evoke tastes). In recent years a number of studies have highlighted the authenticity of synaesthesia and attempted to use the experience to inform us about typical processes in perception and cognition. This Research Topic brings together research on synaesthesia and typical cross modal interactions to discuss the mechanisms of synaesthesia and what it can tell us about typical perceptual processes. Topics include, but are not limited to, the neurocognitive mechanisms that give rise to synaesthesia; the extent to which synaesthesia does / does not share commonalities with typical cross-modal correspondences; broader cognitive and perceptual consequences that are linked to synaesthesia; and perspectives on the origins / defining characteristics of synaesthesia. Citation: Banissy, M., Cohen Kadosh, R., Jonas, C., eds. (2015). Synaesthesia. Lausanne: Frontiers Media. doi: 10.3389/978-2-88919-559-6 SYNAESTHESIA 3 October 2015 | Synaesthesia Frontiers in Psychology Table of Contents 05 Synesthesia: an introduction Michael J. Banissy, Clare Jonas and Roi Cohen Kadosh 08 Why are there different types of synesthete? Julia Simner 11 Synesthesia: a colorful word with a touching sound? Myrto I. Mylopoulos and Tony Ro 20 Synesthesia, at and near its borders Lawrence E. Marks and Catherine M. Mulvenna 24 Training, hypnosis, and drugs: artificial synaesthesia, or artificial paradises? Ophelia Deroy and Charles Spence 39 Mirror-touch and ticker tape experiences in synesthesia Charlotte A. Chun and Jean-Michel Hupé 52 Veridical mapping in savant abilities, absolute pitch, and synesthesia: an autism case study Lucie Bouvet, Sophie Donnadieu, Sylviane Valdois, Chantal Caron, Michelle Dawson and Laurent Mottron 62 Synaesthesia and sexuality: thein fluence of synaesthetic perceptions on sexual experience Janina Nielsen, Tillmann H. C. Kruger, Uwe Hartmann, Torsten Passie, Thorsten Fehr and Markus Zedler 68 Enhanced recognition memory in grapheme-color synaesthesia for different categories of visual stimuli Jamie Ward, Peter Hovard, Alicia Jones and Nicolas Rothen 76 Grapheme-color synaesthesia is associated with a distinct cognitive style Beat Meier and Nicolas Rothen 83 Numerical synesthesia is more than just a symbol-induced phenomenon Limor Gertner, Isabel Arend and Avishai Henik 87 The emotional valence of a conflict: implications from synesthesia Amit Perry and Avishai Henik 95 Affect-related synesthesias: a prospective view on their existence, expression and underlying mechanisms Nele Dael, Guillaume Sierro and Christine Mohr 104 Do you see what I hear? Vantage point preference and visual dominance in a time-space synaesthete Michelle Jarick, Mark T. Stewart, Daniel Smilek and Michael J. Dixon 111 Color and texture associations in voice-induced synesthesia Anja Moos, David Simmons, Julia Simner and Rachel Smith 4 October 2015 | Synaesthesia Frontiers in Psychology 123 Color associations for days and letters across different languages Romke Rouw, Laura Case, Radhika Gosavi and Vilayanur Ramachandran 140 How well do you see what you hear? The acuity of visual-to-auditory sensory substitution Alastair Haigh, David J. Brown, Peter Meijer and Michael J. Proulx 153 Combined structural and functional imaging reveals cortical deactivations in grapheme-color synaesthesia Erik O’Hanlon, Fiona N. Newell and Kevin J. Mitchell 166 A taste for words and sounds: a case of lexical-gustatory and sound-gustatory synesthesia Olympia Colizoli, Jaap M. J. Murre and Romke Rouw 182 The induction of synaesthesia with chemical agents: a systematic review David P . Luke and Devin B. Terhune 194 The role of conceptual knowledge in understanding synesthesia: evaluating contemporary findings from a “hub-and-spokes” perspective Rocco Chiou and Anina N. Rich EDITORIAL published: 15 December 2014 doi: 10.3389/fpsyg.2014.01414 Synesthesia: an introduction Michael J. Banissy 1 *, Clare Jonas 2 and Roi Cohen Kadosh 3 1 Department of Psychology, Goldsmiths, University of London, London, UK 2 School of Psychology, University of East London, London, UK 3 Department of Experimental Psychology, University of Oxford, Oxford, UK *Correspondence: m.banissy@gold.ac.uk Edited and reviewed by: Eddy J. Davelaar, Birkbeck College, UK Keywords: synaesthesia/synesthesia, multisensory, sensory substitution, perception, mirror-touch, grapheme-color synaesthesia, lexical-gustatory synaesthesia, cross-modal correspondence Synesthesia is a rare experience where one property of a stimu- lus evokes a second experience not associated with the first. For example, in lexical-gustatory synesthesia words evoke the expe- rience of tastes (Ward and Simner, 2003). There are at least 60 known variants of synesthesia (Day, 2013), including reports of synesthetic experiences of color (Baron-Cohen et al., 1987), taste (Ward and Simner, 2003), touch (Ward et al., 2008), and sound (Saenz and Koch, 2008). The lower bound prevalence of the con- dition is considered to be approximately 4% (Simner et al., 2006). While synesthetic experiences have been documented since the 1800s (Jewanski et al., 2009), it is only in the last few decades that the authenticity of synesthetic experiences and mechanisms that contribute to them has been explored in depth (Ward, 2013). This resurgence in research has led to developments in our understanding of mechanisms that contribute to the synesthetic experience and the use of synesthesia as a unique experimental preparation to inform us about typical models of cognition and perception (e.g., Cohen Kadosh and Henik, 2007; Simner, 2007; Bargary and Mitchell, 2008; Rouw et al., 2011). This has also resulted in many open questions and debates, several of which are touched upon in this research topic. Specifically, this research topic is focused around the following themes: What constitutes synesthesia and how does it relate to typical cross-modal inter- actions? What mechanisms contribute to synesthetic experiences? Are there broader cognitive and perceptual traits associated with synesthesia, and what mechanisms mediate their relationship? In total, there are 20 articles, each addressing at least one of these themes. WHAT CONSTITUTES SYNESTHESIA AND HOW DOES IT RELATE TO TYPICAL CROSS-MODAL INTERACTIONS? Several authors focus on discussing what synesthesia is and how it relates to typical cross-modal interactions. Mylopoulos and Ro (2013) provide a critical review of methods used for understand- ing and classifying synesthesia and provide a set of markers to aid in distinguishing synesthesia from other psychological phenom- ena. Marks and Mulvenna (2013) provide an interesting opinion article on cases that border on traditional forms of synesthe- sia and discuss whether these forms do or do not constitute forms of synesthesia. Similarly, Deroy and Spence (2013) dis- cuss the notion of “induced synesthesia,” arguing that current attempts to induce synesthesia may not be evidentially linked to developmental synesthesia. Moos et al. (2013) touch on the theme of how synesthe- sia relates to typical cross-modal interactions, reporting findings on color and texture associations in voice induced synesthesia, suggesting common underlying mechanisms in cross-modal asso- ciations between synesthetes and non-synesthetes. Additionally, Rouw et al. (2014) examine the relationship between synesthetic and non-synesthetic cross-modal representations by assessing color associations for days and letters across different languages. Haigh et al. (2013) examine the acuity of visual-to-auditory sensory substation and discuss whether visual imagery evoked by the device is a form of or synthetic synesthesia. Similarly, Simner (2013) contributes to themes of what mechanisms contribute to synesthetic experiences and what constitutes synesthesia by dis- cussing the role of visual mental imagery in different types of synesthetes. In her thoughtful discussion, she suggests that dif- ferences between projector (for whom synesthetic experiences are projected onto external objects) and associator (for whom synes- thetic experiences appear in the “minds eye”) synesthetes may emerge from individual differences in visual mental imagery. WHAT MECHANISMS CONTRIBUTE TO SYNESTHETIC EXPERIENCES? In addition to Simner’s (2013) proposal that synesthetic expe- rience is associated with individual differences in visual men- tal imagery, several other articles also address the mechanisms underlying synesthesia. Gertner et al. (2013) propose that numer- ical synesthesia is more than a symbol-induced phenomenon, and may also be induced by non-symbolic magnitudes. Perry and Henik (2013) report an experiment addressing emotional con- flict sensations evoked in synesthesia when synesthetic photisms and veridical experiences conflict (e.g., when a numeral is pre- sented in the wrong color). They discuss their findings in relation to emotional experience in synesthesia and the extent to which synesthesia may be used as a vehicle to inform us about emo- tional processing in the wider population. Dael et al. (2013) also address affect in synesthesia by providing a thoughtful review article on affect-related synesthesias and underlying mechanisms. Additionally, Jarick et al. (2013) examine vantage point preference and visual dominance in a time-space synesthesia, reporting that their synesthete is able to reverse her perspective on “time.” Moving to neural mechanisms, O’Hanlon et al. (2013) pro- vide a research article examining structural and functional brain correlates of grapheme-color synesthesia. Colizoli et al. (2013) www.frontiersin.org December 2014 | Volume 5 | Article 1414 | 5 Banissy et al. Synesthesia: an introduction report a single case brain imaging study of lexical-gustatory and sound-gustatory synesthesia. Additionally, Luke and Terhune (2013) provide an important review of the induction of synes- thesia with chemical agents, highlighting the potential role of the serotonergic system in synesthesia. Finally Chiou and Rich (2014) discuss the role of concep- tual knowledge in understanding synesthesia. Using a “hub and spokes” approach they present a model of synesthesia in which the inducer and concurrent are linked within a conceptual-level representation. ARE THERE BROADER COGNITIVE AND PERCEPTUAL TRAITS ASSOCIATED WITH SYNESTHESIA, AND WHAT MECHANISMS MEDIATE THEIR RELATIONSHIP? Several authors highlight that synesthetes show different perfor- mance on tasks that are not directly related to their synesthetic experience. For example, Chun and Hupé (2013) address the rela- tionship between synesthesia and other perceptual experiences by examining the prevalence of mirror-touch (Banissy et al., 2009) and ticker tape (Day, 2005) experiences in synesthesia. Bouvet et al. (2014) report an interesting case study involving an autistic individual who possesses savant abilities in addition to absolute pitch and synesthetic-like associations. They discuss the case in relation to the role of veridical mapping in autism, absolute pitch and synesthesia. Nielsen et al. (2013) examine and discuss the influence of synesthetic perceptions on sexual experience. In relation to cognitive differences between synesthetes and non-synesthetes, Ward et al. (2013) address the relationship between grapheme-color synesthesia and enhanced recognition memory by comparing visual recognition memory in grapheme- color and lexical gustatory synesthesia. They show that grapheme- color synesthetes show enhanced visual recognition memory, but this is not found for lexical-gustatory synesthesia. Meier and Rothen (2013) investigate whether synesthesia is associated with a particular cognitive style, and provide evidence to suggest that grapheme-color synesthetes show a preference for a verbal and a specific visual cognitive style. CONCLUDING REMARKS In summary, this Research Topic provides a novel collection of articles on synesthesia addressing a range of issues. We envisage that many of these will provide productive new research areas, and conceptual frameworks for the future study of synesthesia and related processes. REFERENCES Banissy, M. J., Cohen Kadosh, R., Maus, G. W., Walsh, V., and Ward, J. (2009). Prevalence, characteristics and a neurocognitive model of mirror-touch synaes- thesia. Exp. Brain Res . 198, 261–272. doi: 10.1007/s00221-009-1810-9 Bargary, G., and Mitchell, K. J. (2008). Synaesthesia and cortical connectivity. Trends Neurosci. 31, 335–342. doi: 10.1016/j.tins.2008.03.007 Bouvet, L., Donnadieu, S., Valdois, S., Caron, C., Dawson, M., and Mottron, L. 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Cognition 89, 237–261. doi: 10.1016/S0010-0277(03) 00122-7 Conflict of Interest Statement: The authors declare that the research was con- ducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Received: 28 October 2014; accepted: 19 November 2014; published online: 15 December 2014. Citation: Banissy MJ, Jonas C and Cohen Kadosh R (2014) Synesthesia: an introduc- tion. Front. Psychol. 5 :1414. doi: 10.3389/fpsyg.2014.01414 This article was submitted to Cognitive Science, a section of the journal Frontiers in Psychology. Copyright © 2014 Banissy, Jonas and Cohen Kadosh. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, pro- vided the original author(s) or licensor are credited and that the original publi- cation in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. www.frontiersin.org December 2014 | Volume 5 | Article 1414 | 7 OPINION ARTICLE published: 02 September 2013 doi: 10.3389/fpsyg.2013.00558 Why are there different types of synesthete? Julia Simner * Department of Psychology, School of Philosophy, Psychology and Language Sciences, University of Edinburgh, Edinburgh, UK *Correspondence: jsimner@ed.ac.uk Edited by: Clare Jonas, University of East London, UK Reviewed by: Nicolas Rothen, University of Sussex, UK Michelle Jarick, MacEwan University, Canada Keywords: individual differences, visual imagery, synaesthesia, synesthesia, projector, associator For people with synesthesia, sensations in two modalities are experienced when only one is stimulated (e.g., auditory stimuli might trigger colors and sounds). Synesthetes are manifestly different to the general population, but can also be different to each other. First, the con- dition is widely heterogeneous in that 60–150 different manifestations of synes- thesia have been identified (e.g., auditory stimuli might trigger colors, shapes, flavors and so on; Cytowic and Eagleman, 2009). Second, synesthetes can differ on the qual- ity of their synesthetic perceptions even within a given variant. Some experience their synesthetic percepts as being similar in quality to a real-world perceptions (e.g., synesthetic colors might be projected onto external objects and be difficult to disso- ciate from real-world colors) while other synesthetes experience less “veridical” per- cepts (see below). In this opinion piece I ask whether this particular difference— known as the “projector” vs. “associa- tor” distinction—might fall out naturally from another, independent psychological quality. The projector/associator distinction was first phrased (by Dixon et al., 2004) in terms of grapheme-color synesthesia , in which colors are triggered by letters and/or digits. Some grapheme-color synesthetes report that their colors are experienced outside their own body space, projected into the world, and these are termed “projector” synesthetes. Ward and col- leagues further divide this category into two: “surface-projectors” experience color projected onto the written type-face (or more generally, onto the inducing stimulus whatever that might be), and “space- projectors” project color onto some other externalized near-space. In contrast to this external projection, other synesthetes (termed “associators”) can be thought of as “non-projectors” in that their colors exist within their internal mental space. Those associators who claim to see colors in their mind’s eye have been termed “see- associators,” while those who simply claim to know the colors of graphemes without any associated impression of “seeing” at all have been termed “know-associators” (Ward et al., 2007). In other words there is a four-way divide between synesthetes experiencing colors in a way resembling real-world experiences (projected onto the stimulus or into near-space) and those experiencing them “internally” (in the mind’s eye or as a type of propositional association). The claim put forward in this piece is a parsimonious one: that these differences might emerge from an other- wise unrelated individual difference, in the ability to form a visual mental image. Visual imagery—the mental construc- tion of a scene-like object—is known to vary across individuals (e.g., Marks, 1973). At the upper extreme end there are indi- viduals with “eidetic” imagery who report strongly evoked mental images with an almost veridical quality. At the opposite end of the healthy spectrum are those who report being unable to form mental images at all. Taking these latter into considera- tion, and assuming a prevalence of synes- thesia of at least 4% (Simner et al., 2006), we might assume—all other things being equal—that 4% of people with poor or no imagery abilities will have synesthesia. The proposal here is that these individuals are precisely those termed “know associators.” Equally, those with extreme imagery abil- ities might be those we recognise as pro- jectors, because their high imagery allows their synesthetic associations to become “scene-like” to an extreme extent. Note that this view (if it were correct) would rely on the assumption that imagery and synes- thesia are independent qualities and I now compare this view with current thinking in the literature. Some researchers have proposed that synesthetes are in fact characterized, as a group, by superior mental imagery (e.g., Barnett and Newell, 2008; Price, 2009a,b), and some have gone so far as to ask whether some visual synesthesias may be nothing more than extreme visual imagery (Galton, 1880; Phillips, 1897; Price, 2009a). One well-phrased expres- sion of a possible link between synesthesia and imagery is that normal cross-modal sensations (say, between numbers and space) may enter into consciousness only if the individual has heightened imagery (Eagleman, 2009; Price, 2009a), and this is precisely when he/she would become considered a “synesthete” (Simner, 2012). (The resultant synesthesia in this exam- ple would be sequence-space synesthesia — in which sequences such as numbers are consciously experienced in spatial arrays). In contrast with this view, others have sug- gested that synesthesia and imagery might be quasi-independent in that synesthetes might vary in their imagery ability, and hence some could have relatively weak imagery (see Grossenbacher and Lovelace, 2001, who put this view in passing). Questions about synesthesia and imagery have also been tested empiri- cally. Barnett and Newell (2008) showed that ( n = 38) synesthetes with colored language (e.g., grapheme-color synesthe- sia) report significantly stronger vivid everyday mental images than controls, in a self-report questionnaire ( Vividness of Mental Imagery Questionnaire , VVIQ , Marks, 1973). Meier and Rothen (under review) show similarly for ( n = 24) grapheme-color synesthetes using the www.frontiersin.org September 2013 | Volume 4 | Article 558 | 8 Simner Why are there different types of synesthete? Verbalizer-Visualizer Questionnaire (VVQ; Kirby et al., 1988). And the same has been shown by Price (2009a) testing 12 sequence-space synesthetes, who outper- formed controls in two questionnaires of visual imagery ( Subjective Use of Imagery Scale , SUIS ; Reisberg et al., 2003; and the visual [but not spatial] component of the Object–Spatial Imagery Questionnaire , OSIQ ; Blajenkova et al., 2006). However, there has been some variation in the con- sistency of this self-reported superiority in imagery. Seron et al. (1992) failed to find higher than average visual imagery in 26 sequence-space synesthetes, using Paivio’s Individual Difference Questionnaire (IDQ; Paivio, 1971). Furthermore, Spiller and Jansari (2008) failed to find a group differ- ence in the VVIQ between six grapheme- color synesthetes and matched controls, although this may be due to low power. In addition to poor consistency across synesthesia studies showing self-reported superiority in imagery, there has also been some variation in the extent to which this has translated into behavioral sup- port. Despite no self-reported differences, grapheme-color synesthetes in Spiller and Jansari (2008) out-performed controls on a task that relied on mental imagery (imag- ing a grapheme within a divided circle and assessing which segment contained the majority of that image). Another study also found behavior suggesting synesthetes have superior imagery (e.g., sequence- space in 3D mental rotation; Simner et al., 2009). However, their group size was small ( n = 5) and this effect was not replicated in a larger sample of nine synesthetes with a similar (sequence-space) variant (Rizza and Price, 2012). Nonetheless, Simner et al. tested somewhat unusual synesthetes with an unusually large array of synes- thetic forms (e.g., for minutes, hours, days, months, numerals, letters, tempera- ture etc.; mean = 7.0 forms; SD = 2 3). In contrast, Rizza and Price’s subjects required only two forms to be selected for study. Superior performance in behav- ioral imagery tasks might therefore be tied to “superior” (more extreme) synesthe- sia. Nonetheless, a superiority in men- tal rotation for ( n = 15) sequence-space synesthetes has now also been replicated by Brang et al. (2013; in 2D rotation), although across all three studies, we must conclude that the effect does not appear to be robust enough to survive repeated repli- cation, especially where synesthetes might vary in their “strength.” Finally, Spiller and Jansari found no correlation between their self-reported imagery scores and a behavioral visual imagery task, suggesting that self-report may not always reflect performance in lab- based imagery tests. Furthermore, Rizza and Price (2012) point out that mental rotation has not previously correlated with self-report in visual imagery, but rather spatial imagery, even though synesthetes do not report higher than average spatial skills (Rizza and Price, 2012; Meier and Rothen, under review). However, as Logie et al. (2011; p. 3072) “subjective reports [of mental imagery] do not always corre- late with performance on mental imagery tasks . . . [perhaps because] people have poor insight into their mental operations when rating them.” From these somewhat conflicting findings we can draw the following con- clusions: although grapheme-color and sequence-space synesthetes have reported higher imagery in self-report visual imagery questionnaires, this self-report advantage has not been found consistently across studies of grapheme-color synes- thetes (e.g., Barnett and Newell, 2008 vs. Spiller and Jansari, 2008 and Seron et al., 1992) and nor has it consistently translated into behavioral superiority for sequence- space synesthetes in mental rotation (e.g., Simner et al., 2009; Brang et al., 2013 vs. Rizza and Price, 2012). Furthermore, there is conflict both across and within stud- ies between the predictions of self-report imagery questionnaires and behavioral tests such as imaging graphemes (for grapheme-color synesthetes; e.g., Spiller and Jansari, 2008) and mental rotation (for sequence-space synesthetes; e.g., Rizza and Price, 2012 vs. Simner et al., 2009 and Brang et al., 2013). Where conflicts arise across different cohorts of synesthetes with the same variant, we might conclude either that low power in some studies may be to blame, or subtle differences in methodologies, or indeed that meaningful individual differences in imagery might exist. In other words, if conflicts across studies represent genuine differences in imagery across synesthetes, then imagery and synesthesia may be either partially or fully independent. If synesthesia and imagery are only partially independent, we might conclude that whatever causes synesthesia could perhaps sometimes have repercussions for high imagery, or vice versa, but that high imagery is not a necessary component of synesthesia. This would certainly be compatible with findings in functional magnetic resonance imaging (fMRI) that synesthetic colors are experienced with at least some differences in regions compared to the colors within the mental imagery of non-synesthetes (Rich et al., 2006). Partial independence could also explain why some synesthetes show no significant advan- tage in tests/questionnaires of imagery: if synesthesia and high imagery only tend to be linked, some cases would exist where the synesthete’s imagery was low. However the tendency itself would also explain why there is an overall trend toward finding higher imagery across the body of litera- ture as a whole (e.g., Barnett and Newell, 2008; Spiller and Jansari, 2008; Meier and Rothen, under review). But let us now consider the possibil- ity that these skills may be fully indepen- dent. If correct, this theory would smartly explain why some studies find no imagery advantages in their synesthetes. However, it would also need to explain why some synesthetes do score highly in imagery, and perhaps more often than we might expect by chance alone. Barnett and Newell (2008) and others have suggested that high imagery is a general characteristic of synes- thesia but I would suggest it may simply be a characteristic of those synesthetes that self-report for testing. What then is dif- ferent about these individuals? One con- sistent quality of all participants in the imagery studies reviewed here is that all were fully aware of their synesthesia (hence their self-referral and/or affirmations of synesthesia), but importantly, this need not be true of all synesthetes. It is reason- able to propose that an individual’s aware- ness of synesthesia might be heightened if individual differences in imagery make synesthetic colors highly image-able for some synesthetes (but not others—who instead might simply “know” their asso- ciations at some propositional level). In other words, I suggest that individual and randomly dispersed variations in mental imagery within the synesthesia population would lead to some synesthetes having Frontiers in Psychology | Cognitive Science September 2013 | Volume 4 | Article 558 | 9 Simner Why are there different types of synesthete? high imagery and therefore highly imaged synesthetic colors. This in turn could lead to enhanced conscious awareness, and consequently a greater likelihood of self- referral for scientific studies. This pro- posal is in opposition to the view that synesthetes overall are necessarily high imagers as a sin qua non and/or that synesthesia is nothing more than high mental imagery. (Indeed, this same self- referral argument might hold equally over other hypotheses, such as whether synesthetes have enhanced memory; e.g., Rothen et al., 2012). In summary, I have proposed that empirical evidence is currently equivocal for synesthetes as high imagers. I have also suggested that trends toward high imagery in published studies might arise from a recruitment bias in which high-imaging synesthetes are more likely to self-refer. If high imagery is neither sufficient nor nec- essary for synesthesia, this would explain why some studies fail to show superior imagery, and would also nicely capture why some synesthetes describe their colors (or other sensations) as simply “known.” Put differently, for those without the abil- ity to form a mental image, there would be no visual-like quality to their synesthetic sensations. This theory would also explain why other synesthetes do, in contrast, describe their colors as highly imaged, even eidetically so. My proposal sits along- side another theory, by Ward et al. (2007), that other differences within this same spectrum (e.g., surface vs. near-space pro- jectors) could also fall out naturally from other a priori individual differences (this time from differences in spatial reference frames). In future studies it will be impor- tant to establish whether trends found in smaller populations are true even when larger numbers are assessed. It is also important to look at recruitment meth- ods since self-referred samples might be expected to show stronger imagery than cohorts more randomly sampled. Finally, future studies might seek a standardized assessment of projector/associators, given that previous efforts to classify individ- uals have shown differences across labs (c.f., Dixon et al., 2004; Ward et al., 2007) and even over time (Edquist et al., 2006). A useful classification would also extend to all forms of synesthesia, including vari- ants (e.g., sequence-space) in which the projector/associator distinction has been recognized thus far only in self-report. REFERENCES Barnett, K. J., and Newell, F. N. (2008). Synaesthesia is associated with enhanced, self-rated visual imagery. Conscious. Cogn.